In an x-ray computed tomography (“CT”) system, an x-ray source projects a fan-shaped or cone-shaped beam of x-rays that is collimated to lie within an x-y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object, and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all of the detectors are acquired separately to produce a transmission profile at a particular view angle.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane, and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view,” and a “scan” of the object includes a set of views acquired at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to reconstruct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers,” or “Hounsfield units,” which are used to control the brightness of a corresponding pixel on a display.
In recent years, a dramatic improvement in multi-detector CT technology has occurred. The ability to noninvasively image the coronary artery lumen and wall and obtain information on the presence, severity, and characteristics of coronary artery disease (CAD) became an attractive addition to currently available diagnostic tools (such as, for example, nuclear perfusion imaging or invasive selective coronary angiography) for patient workup. For example, according to several studies, a common type of cardiac CT imaging technique referred to as the coronary CT angiography (CCTA) provides substantially high diagnostics accuracy to detect stenoses (with sensitivity well in excess of about 73 percent and specificity of about 90 to about 97 percent). At first, CCTA was performed with a continuous level of tube current and with concurrent recording of the ECG, such that one can retrospectively select cardiac phases of interest within diastole or systole. To reduce radiation dose, further advances of CCTA technique included the modulation of the tube current for different cardiac phases, as a result of which the X-ray exposure of the object of imaging may be limited to chosen cardiac phases (single phase, if necessary). A cardiac cycle is conventionally understood as the time between two heart beats. To identify the heart beats, CCTA uses the electrocardiographic (ECG) signal where the cardiac cycle is represented by the time between two R waves. For practical purposes, in CCTA the cardiac cycle is typically divided into 20 or 10 phases (corresponding, respectively, to every 5 or 10% of the R-R interval). The systole typically includes cardiac phases corresponding to a range of the R-R-interval from 0% to about 40%, while the diastole typically includes cardiac phases corresponding to a range of the R-R interval from about 40% to about 100%.
Depending on clinical needs, physicians can be interested in one or multiple cardiac phases. After the CCTA exam, it is possible to obtain a volume of the whole heart comprising the myocardium, chambers, cardiac tissue and its vessels for all cardiac phases. In currently employed ECG-triggered CCTA methodologies, a choice has to be made between a) reducing the dose of radiation exposure of the patient by limiting the optimal CT acquisition time window (which window corresponds to the maximum tube current) to only the desired cardiac phase(s), as a result drastically reducing the image quality of cardiac phases out of the phase selected and also lowering the probability of an optimal scan when heart rate changes or patient moves, and b) maintaining the data acquisition at high levels of CT-tube current and, therefore, keeping high image quality for all cardiac phases while increasing robustness to instabilities of heart rate, but, at the same time, substantially increasing radiation dose to which the object of imaging is being exposed. In the case where imaging data was acquired during all cardiac cycles but only one optimal cardiac phase is reconstructed for diagnostic purposes, a portion of the emitted CT radiation is wasted, for the purposes of imaging.
Accordingly, there remains a need in a method and system facilitating optimization of the cardiac imaging process in general and the CCTA process in particular.